We have isolated Simian immunodeficiency virus (SIV) from 3 fetal asymptomatically infected African green monkeys. Our SIV AGM- isolates have the standard characteristics reported for SIV's isolated from other simian genera and of HIV: lentivirus morphology and mode of replication, growth in CD4 bearing cells and Mg++ dependent reverse transcriptase. However, replication of these SIV AGM isolates did not occur in human peripheral blood lymphocytes (PBL) nor in the human CD4 bearing lymphoid tumor cell lines, H9 and Hut 78, as has been reported for SIV's from rhesus and sooty mangabey monkeys. Our isolates also appear to be less virulent for rhesus monkeys than SIV's from rhesus and sooty mangabey monkeys. Rhesus monkeys inoculated with SIV AGM became infected but within several weeks infectious virus could not be detected in serum or co-cultured PBL from these animals. These animals, nevertheless, were latently infected as SIV AGM could be activated by secondary infection of these animals with SAIDS retrovirus type 1, a Mason- Pfizer-like type D retrovirus. Several of the dually infected animals developed fatal lymphomas, a disease trait not experienced by animals singly infected. Our SIV AGM/SRV-1 model may be very useful in elucidating factors which lead to activation of latent HIV-1 infections leading to AIDS.